JSON to Kotlin

About the JSON to Kotlin

The JSON to Kotlin converter generates Kotlin data classes from a JSON sample, annotated for kotlinx.serialization by default. Keys become camelCase properties with @SerialName where they differ, arrays become List<T>, and keys missing from some items become nullable with a default of null.

Conversion happens in your browser, so your files and records stay private.

How to use the JSON to Kotlin

1. Paste a JSON sample and set the root class name.
2. Choose kotlinx.serialization annotations or plain data classes.
3. Copy the code and decode with Json.decodeFromString<User>(text).

Example

{
  "id": 7,
  "fullName": "Grace Hopper",
  "email": null,
  "roles": ["admin"],
  "team": { "name": "Compilers", "size": 4 },
  "projects": [
    { "name": "COBOL", "year": 1959 },
    { "name": "A-0", "year": 1952, "archived": true }
  ]
}

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.JsonElement

@Serializable
data class User(
    val id: Long,
    val fullName: String,
    val email: JsonElement?,
    val roles: List<String>,
    val team: Team,
    val projects: List<Project>,
)

@Serializable
data class Project(
    val name: String,
    val year: Long,
    val archived: Boolean? = null,
)

@Serializable
data class Team(
    val name: String,
    val size: Long,
)

Common mistakes

• Missing keys without defaults. kotlinx.serialization throws MissingFieldException when a non-optional property is absent. Keep = null on properties that can be missing.
• Unknown keys in the input. By default, extra keys in the JSON cause an error. Decode with Json { ignoreUnknownKeys = true } when the API may add fields.

Works well with the JSON to Kotlin

• JSON Formatter: tidy and check the JSON before generating types.
• JSON to TypeScript: generate matching TypeScript interfaces for the front end.
• JSON Validator: find syntax errors in the sample first.

Frequently asked questions